1. Field of the Invention
The present invention relates to the field of turbine engines and refers to a decoupling sleeve of which the function is to ensure the transmission of torque between an engine shaft and a coaxial shaft which it drives.
2. Description of Related Art
In the field of turbine engines, to ensure the start-up of a turbojet engine, for example, a starter means is used such as a turbine starter which drives the shaft of the engine to be started in rotation until said shaft functions independently. A known starter is driven by air. The pneumatic power issuing from a different engine during operation or from an auxiliary power unit is transformed into mechanical power by means of a turbine. A reduction gear reduces the speed of the turbine to an acceptable value for the driven device such as the accessory gear box of the turbojet engine. The kinematic chain comprises, in particular, a free wheel which allows the starter to be disengaged once the engine has started up.
For safety reasons, a coupling member is arranged between the starter shaft and the shaft of the machine to be started which has means for decoupling the two shafts to prevent damage to one or other of the machines which could arise in the case of malfunction. This coupling member is designated hereinafter by the term decoupling sleeve.
The functions generally assigned to this sleeve are as follows:
During forward operation, it transmits a starter torque from the starter to the shaft of the machine which is driven, such as a shaft of the accessory gear box in a gas turbine engine,                it limits the torque transmitted in this forward direction.        it prevents the free wheel from jamming which would be damaging to the starter, by preventing the starter from being driven at a specific speed in the reverse direction.        
It is apparent that this last function is difficult to ensure as the levels of reverse torque for the sleeve to function are low. Moreover, it is difficult to fulfill this condition without making the sleeve fragile.
Devices are known which ensure this decoupling function when reverse torque occurs.
Patent U.S. Pat. No. 4,871,296 discloses a device consisting of two splined shafts joined by dog clutch jaws. In the forward direction of torque transmission, the dog clutch jaws drive the two shafts. When a reverse torque predetermined by the compression of a spring is attained, the dog clutch jaws are separated from one another. When a ring, mounted on a rod attached to the driving shaft, abuts a planar portion attached to the driven shaft, it prevents the two shafts from continuing to separate until the reverse torque is sufficient to cause the rod to fracture (a fracture zone has been provided on the rod for this purpose). The decoupling therefore takes place when the frangible rod is broken in traction.
Patent U.S. Pat. No. 6,059,085 discloses a similar device. The sleeve consists of two splined shafts joined by dog clutch jaws, by means of which the torque in the forward direction is transmitted. When a reverse torque predetermined by the compression of the spring is attained, the dog clutch jaws are separated from one another. A pin mounted transversely on a coaxial rod and attached to the driving shaft, abuts in the slots of a ring. This ring is attached to the driven shaft. A preferred fracture zone is provided on the rod and the decoupling takes place when the frangible rod is broken in torsion.
It is observed that this type of solution with the fracture either in traction or in torsion of a frangible rod transmitting the torques, exhibits a certain lack of robustness. More specifically, the fracture zones provided on the rods, whether in traction or torsion, are relatively thin.
They are sensitive:                to misalignments of the splines which cause locking torques liable to fatigue the metal;        to parasitic reverse torques induced by the machine which is driven, such as the accessory gear box in a gas turbine engine.        
The fracture due to fatigue of the frangible rod may therefore induce untimely fractures of the decoupling sleeve which reduces reliability during the operation of the starter.